The present invention relates to a light-sensitive printing plate comprising a support, a light-sensitive layer and a superimposed ink-repellent layer of a silicone elastomer.
Printing plates of this type are known, for example, from German Offenlegungsschrift 16 71 637. The light-sensitive layers used therein are preferably those based on polycondensation products of aromatic diazonium salts and formaldehyde. By exposure, these layers become insoluble in the developer and can be developed with aqueous solutions. During development, those areas of the silicone elastomer layer which are located above the soluble, unhardened areas of the light-sensitive layer are removed together with these soluble areas, even though they are insoluble in the developer. It is necessary, however, to add sufficient quantities of organic solvents to the developer, so that the silicone elastomer exposed to the developer becomes swollen and thus softened. For example, mixtures of isopropanol and water are used for development.
The silicone elastomers used for this purpose are single-component elastomers which require moisture and a considerable length of time in order to harden after application of the layer. Such single-component silicone elastomers are usually polysiloxanes, for example dimethylpolysiloxanes which contain terminal acetyl, oxime, hydroxyl or amino groups.
German Offenlegungsschrift 23 23 972 discloses a similar plate with a single-component silicone elastomer, wherein the light-sensitive layer contains a reaction product, obtained from a diphenylamine-4diazonium salt/formaldehyde condensation product and 2-hydroxy-4-methoxy-benzophenone-5-sulfonic acid, and, if appropriate, a binder. Since the solubility of the diazonium salt precipitation product is very low, both the preparation and the development of the plate are difficult.
In German Offenlegungsschrift 23 57 871, presensitized plates for waterless offset printing are described which contain a multi-component silicone elastomer layer. In addition to organopolysiloxanes, these layers contain, for example, low-molecular weight silanes which are capable of reacting with functional end groups of the polysiloxanes with crosslinking in the absence of moisture. These multi-component systems can crosslink by means of addition reactions or condensation reactions. In the addition type, as described in the above-mentioned German Offenlegungsschrift, for example, terminal alkenyl groups react with Si-H groups in the presence of catalysts at an elevated temperature.
The addition type yields plates that are highly ink-repellent and readily developable. However, the layers are frequently sensitive to scratching so that, upon handling a plate in the printing machine, scratches occur on the silicone surface. These scratches appear in the impression and thus render the printing plate useless. In the condensation-crosslinking systems, terminal functional groups of polydiorganosiloxanes react with functional groups of silanes or oligosiloxanes which act as crosslinking agents, in most cases in the presence of catalysts.
The printing plates obtained with these silicone rubber layers show good ink repellency in the non-image areas and, as a result of a high crosslinking density, excellent scratch resistance of the ink-repellent silicone layer. However, printing plates provided with layers of this kind are very difficult to develop and resolution of the image elements is insufficient.
According to German Offenlegungsschrift 29 41 254, a so-called primer is applied in a thin layer between the light-sensitive layer and silicone rubber layer, in order to improve the adhesion between these layers. The silicone primers or silicone coupling agents mentioned include, for example, alkyl, aminoalkyl and alkoxy silanes.
These substances merely serve to enhance the adhesion of the silicone rubber to the light-sensitive layer.